4T Score Calculator | HIT

The 4T score aggregates points across four distinct clinical criteria, each scored from 0 to 2, yielding a total composite score between 0 and 8. The mathematical evaluation follows the equation: Total Score = Thrombocytopenia (derived from the platelet count fall and nadir) + Timing of platelet drop + Thrombosis or other sequelae + Other causes of thrombocytopenia. A higher cumulative score correlates with an increased statistical probability of immune-mediated HIT, dictating whether clinicians should immediately cease heparin and initiate alternative non-heparin anticoagulation.

Consider a 65-year-old hospitalized patient who developed a new deep vein thrombosis 8 days after starting unfractionated heparin therapy. Their baseline platelet count was 250,000/mcL, and their platelet nadir dropped to 85,000/mcL, representing a 66% decrease. Evaluating the individual components: The platelet fall is greater than 50% with a clear nadir (Score = 2). The timing of the platelet drop occurs precisely 5 to 14 days post-exposure (Score = 2). The new thrombosis is documented (Score = 2). Finally, there are no other apparent causes for the thrombocytopenia (Score = 2). Summing these variables (2 + 2 + 2 + 2) yields a total 4T score of 8, indicating a high probability of HIT requiring urgent intervention.

When utilizing the 4T score, always rely on the percentage drop from the patient's pre-heparin baseline rather than just the absolute platelet count. Be thorough when assessing 'other causes,' as sepsis, medications, and disseminated intravascular coagulation can confound thrombocytopenia evaluations. Remember that a low 4T score (0-3) carries a high negative predictive value, making HIT very unlikely, whereas intermediate or high scores mandate immediate antibody laboratory testing such as a PF4 ELISA.